The present invention relates to a spark plug and an ignition apparatus which are provided for an internal combustion engine of an automotive vehicle to ignite the fuel mixture introduced into a combustion chamber.
Conventionally known for improving the combustibility of fuel mixture is, for example, a multi-ignition technique using a plurality of spark plugs provided in the same combustion chamber or an intake air amount increasing technique using an intake port having an enlarged diameter.
According to these combustibility improving techniques, a very limited space is available for a spark plug. Under such circumstances, downsizing of spark plug as well as downsizing of ignition coil are keys to effectively utilize a limited combustion space.
An effective method for realizing the downsizing of spark plug and ignition coil is to reduce a discharge gap, i.e., a spatial clearance between a center electrode and a ground electrode disposed in an opposed relationship. This method is effective in reducing a discharge voltage, i.e., a voltage required for igniting fuel mixture.
However, according to the research and development of inventors of this invention, simply narrowing a discharge gap will result in worse ignitability of fuel mixture because the electrodes tend to obstruct the growth of a flame kernel caused in a narrowed discharge gap. In other words, the electrodes absorb the heat of a flame before the flame kernel grows sufficiently.
In view of the foregoing problems of the prior art, the present invention has an object to provide a spark plug that has a narrowed discharge gap but is capable of maintaining adequate ignitability and thus capable of realizing the downsizing of spark plug.
Furthermore, the present invention has an object to provide an ignition apparatus using this spark plug.
According to the inventors of the present invention, there is a tendency that the ignitability is significantly worsened in a discharge gap less than 1.1 mm. Knowing the difficulty in maintaining adequate ignitability in such a narrowed discharge gap, the inventors have zealously challenged to optimize the size (i.e., a diameter or the like) of the discharging electrodes for realizing an excellent spark plug capable of maintaining sufficient ignitability with a narrowed discharge gap.
To accomplish the above and other related objects, the present invention provides a first spark plug having a center electrode and a ground electrode spaced in an opposed relationship, wherein at least one of the center electrode and the ground electrode is made of an iridium alloy, a discharge gap between the center electrode and the ground electrode is less than 1.1 mm, and cross sections of the center electrode and the ground electrode are equal to or smaller than 0.95 mm2 in a spherical region where a distance from a midpoint of the discharge gap is within 0.6 mm.
According to the first spark plug of the present invention, at least one of the center electrode and the ground electrode disposed in an opposed relationship is made of an iridium alloy. Due to its excellent durability, the iridium alloy can improve the durability of the spark electrode.
Furthermore, according to the intention of inventors, the first spark plug of the present invention is based on a downsized spark plug having a discharge gap less than 1.1 mm. As an optimized result, the inventors of the present invention have concluded that it becomes possible to maintain satisfactory ignitability when the cross sections of the center electrode and the ground electrode are equal to or smaller than 0.95 mm2 in a spherical region where the distance from the midpoint of the discharge gap is within 0.6 mm.
According to a preferable embodiment, the cross section of the ground electrode is smaller than the cross section of the center electrode.
The flame kernel appears in a discharge gap and grows in a direction advancing toward a combustion chamber. In other words, the flame kernel encounters and collides with the ground electrode in the process of growth. In view of this fact, it is preferable that the cross section of the ground electrode is smaller than the cross section of the center electrode so as to prevent the ground electrode from obstructing the growth of flame.
The present invention provides a first ignition apparatus equipped with the above-described first spark plug, wherein an ignition power source is provided to apply a first voltage to one of the center electrode and the ground electrode which is made of an iridium alloy and to apply a second voltage higher than the first voltage to the other of the center electrode and the ground electrode.
According to a preferable embodiment, a negative electrode (i.e., one of the center electrode and the ground electrode) is made of an iridium alloy and a positive electrode (i.e., the other of the center electrode and the ground electrode) is made of an iridium alloy or other metal. Due to its small work function, the iridium alloy has a nature of easily releasing the electrons. Thus, forming the negative electrode by an iridium alloy makes it possible to cause electrons to easily depart from the negative electrode. Accordingly, it becomes possible to suppress or reduce a required discharge voltage. Thus, the present invention provides an ignition apparatus that is preferably employed to downsize an ignition coil.
Furthermore, according to a preferable embodiment of the first spark plug, the center electrode is rodlike and supported by a cylindrical metal fitting so that a distal end portion of the center electrode protrudes from one end of the metal fitting. The ground electrode is rodlike and fixed to a support member rigidly connected to the metal fitting. The support member has a proximal portion extending straight in parallel with an axis of the center electrode from the one end of the metal fitting and has a distal portion bent at a center of the support member and supporting the ground electrode. A free end of a ground electrode opposes to a side surface of the center electrode. And, the free end of the ground electrode is closest to the center electrode.
This arrangement is characterized in that the support member is bent at its center so that the free end of the ground electrode opposes to a side surface of the center electrode to form the discharge gap therebetween. Thus, it becomes possible to reduce the substantial length from the free end of the ground electrode to the metal fitting. The heat path extending along the support member is so short that heat of ground electrode can be smoothly released to the metal fitting. Ignitability can be adequately maintained. Furthermore, according to this arrangement, the free end of the rodlike ground electrode is closest to the center electrode. This arrangement makes a target of discharge clear and focused in a narrow region when seen from the center electrode. Ignitability can be improved.
Furthermore, the present invention provides a second spark plug having a center electrode and a ground electrode, wherein the ground electrode is a trapezoidal shape with a short side closer to the center electrode in a cross section normal to a central axis of the ground electrode. A length of the short side is in a range from 0.2 mm to 0.7 mm, and an apical angle of the trapezoidal shape at the short side is equal to or smaller than 135xc2x0.
According to the second spark plug, the short side of the trapezoidal ground electrode faces the center electrode so as to form a discharge gap therebetween. The length of the short side is in the range from 0.2 mm to 0.7 mm. A discharge surface thus formed is capable of suppressing the increase of discharge voltage without obstructing the growth of a flame kernel. Having the apical angle equal to or smaller than 135xc2x0 at the short side of the trapezoidal ground electrode is effective to provide an adequate flame guide surface on the ground electrode inclined toward the combustion chamber. Thus, the flame kernel can expand smoothly along the slope of a trapezoidal ground electrode. Thus, the growth of flame kernel is optimized. The present invention provides a spark plug capable of assuring adequate ignitability and thereby realizing the downsizing of a spark plug even when a discharge gap is narrowed.
The present invention provides a third spark plug having a center electrode and a ground electrode spaced in an opposed relationship, wherein the ground electrode is made of a noble metallic alloy having a work function equal to or less than 5 eV, a discharge gap R1 is provided between the center electrode and the ground electrode, and a cross section of the ground electrode is equal to or smaller than 0.95 mm2 in a spherical region where a distance R2 from a midpoint of the discharge gap R1 is within xc2xdxc3x97R1+0.1 mm.
Thermoelectrons emitted from the ground electrode surround a discharging surface of the ground electrode. The electric field strength or intensity (i.e., gradient of electric potential) is locally increased in the vicinity of the discharging surface of the ground electrode. In other words, the distribution of discharge energy can be densified in the vicinity of the discharging surface of ground electrode. As a result, a flame kernel appears at an offset position closer to the ground electrode with respect to the midpoint of the discharge gap. As the cross section of the ground electrode is sufficiently small, the flame kernel can smoothly grow toward the center of the combustion chamber. Higher ignitability can be assured.
Furthermore, by forming the ground electrode by a noble metallic alloy having a work function equal to or less than 5 eV, and by restricting the cross section of the ground electrode to be equal to or smaller than 0.95 mm2 in a predetermined region, it becomes possible to maintain the surface temperature of the ground electrode to a level capable of releasing the thermoelectrons even when the combustion gas temperature is low (for example, in an idling condition). Thus, the present invention can assure excellent ignitability of a spark plug in the entire engine driving conditions.
According to a preferable embodiment of the present invention, the center electrode and the ground electrode have cross sections in a range from 0.13 mm2 to 0.5 mm2 when positioned in the spherical region where the distance R2 from the midpoint is within xc2xdxc3x97R1+0.1 mm.
This arrangement is effective to improve the heat and acid resistivity as well as the ignitability.
Furthermore, to eliminate impurities depositing between the center electrode and the ground electrode, it is preferable that the discharge distance R1 is equal to or larger than 0.3 mm.
It is also preferable that a ridge having a curvature radius equal to or less than 0.2 mm is provided at a distal end of the ground electrode.
An edge effect of the ridge enlarges the electric field strength in the vicinity of the distal end of the ground electrode.
It is also preferable that at least one of the center electrode and the ground electrode has a distal end configured into a spherical shape.
When a ratio of surface to volume of the electrode is small, it becomes possible to prevent the electrode from absorbing the heat of a flame.
Moreover, the present invention provides a second ignition apparatus equipped with the above-described second spark plug, wherein an ignition power source is provided to apply a positive voltage to the center electrode during an ignition discharge.
The thermoelectrons settling in the vicinity of the discharging surface of ground electrode have a function of locally decreasing the electric potential. Thus, it becomes possible to increase a substantial voltage applied between the center electrode and the ground electrode. The discharge operation can be stabilized.